1. Field of the Invention
The present invention relates to a power regulator for supplying power to a load efficiently, and more particularly to a power regulator that achieves this by using an active filter.
2. Description of Related Art
As shown in FIG. 6, a power supply for various electrical appliances acquires a DC voltage from a commercial AC line using a rectifier circuit 91 and a large capacity smoothing capacitor 92. The rectifier circuit 91 instantaneously grabs power from the AC line when the AC voltage exceeds a capacitor charged voltage. An inverter 93 operating as a switching circuit converts the DC voltage charged by the capacitor to an AC voltage, and supplies it to a load 94. As clearly seen from FIG. 7, the power is captured only around the peaks of the AC line voltage as illustrated by a solid waveform. Therefore, the effective power is considerably lower than the apparent power of the AC power supply, resulting in a rather low power factor of the electrical appliances of about 50-70%.
In view of this, a power regulator is proposed whose power factor is improved using an active filter 100 as shown in FIG. 8. In FIG. 8, the reference numeral 101 designates an AC power supply; 102 designates a DC power source using a full-wave rectifier; 103 designates a switching device connected in parallel with the DC power source 102 via a coil 104 of the active filter 100 at the output side of the DC power source 102; 105 designates a charge-and-discharge capacitor connected in parallel with the switching device 103 through a backflow preventing diode 106; 107 designates an inverter for supplying a fluorescent lamp 108 with an AC voltage generated by switching the charges of the charge-and-discharge capacitor 105; 109 designates a zero-cross detector for detecting zero-cross points of the AC current; 110 designates an input current detector for detecting consumed current; 111 designates a microcomputer that receives detection signals from the zero-cross detector 109 and input current detector 110; and 112 designates a driver for bringing the switching device 103 into conduction in response to a control signal from the microcomputer 111.
Next, the operation of the conventional power regulator will be described.
The DC power source 102 converts the AC output of the AC power supply 101 into a DC output to charge the charge-and-discharge capacitor 105 via the coil 104 and backflow preventing diode 106. Then, the inverter 107 generates a high frequency AC voltage by switching the voltage charged in the charge-and-discharge capacitor 105, and supplies it to the fluorescent lamp 108.
During charging of the charge-and-discharge capacitor 105, no current will flow from the AC power supply 101 to the charge-and-discharge capacitor 105 in the periods in which the output voltage of the AC power supply 101 is lower than the voltage charged in the charge-and-discharge capacitor 105. In other words, the charge current flows only when the output voltage of the AC power supply 101 is higher than the voltage charged in the charge-and-discharge capacitor 105. This will result in a lower power factor and higher frequency noise.
In view of this, Japanese patent application laid-open No. 5-300794/1993 proposes a technique in which the microcomputer 111 carries out its control as follows. First, the zero-cross detector 109 detects the zero-cross of the AC input, and the microcomputer 111 detects the instantaneous values of the input current successively over the half wave from the zero-cross point. The microcomputer 111 compares the instantaneous values with prestored reference values, regulates the ON time of the switching signal of the switching device 103 in response to the difference of these values, and carries out the ON/OFF control of the switching device 103 through the driver 112 such that the input current (that is, the inductor current) can take a sinusoidal waveform in phase with the input voltage as shown in FIG. 9.
Alternatively, active filter control (for improving power factor) is carried out by a circuit composed of a switching device controlled by a dedicated IC, an inductance and a diode such that the charging is performed even in the duration in which the AC power supply voltage is less than the DC power supply voltage.
In the conventional power regulator with the foregoing arrangement, the periods of the control pulses for bringing the switching device 103 into conduction are predetermined values. This presents a problem of requiring a high performance microcomputer 111 that can instantaneously calculate the consumed current and supply voltage to vary the pulse periods of the control pulses in accordance with the temperature change, circuit constant errors and the like. Furthermore, because bringing the switching device 103 into conduction while the residual field of the coil 104 is not yet sufficiently reduced will cause a short circuit which will cause energy loss and harmonic noise, the pulse periods must be set with considerable margin. This presents a problem of reducing the effect of the power factor improvement.